The invention relates to a method of producing an MRI image in an MRI apparatus, wherein a gradient field is generated by a gradient system, wherein the desired variation with respect to time of the gradient field is prescribed by a gradient demand, and wherein a quantity representing the effective conduction state of the gradient system is determined during the generation of gradient pulses forming the gradient field.
Such a method is known from Japanese patent abstract No. JP 07303619. In an MRI apparatus, the desired variation of the gradient pulses required for a recording is generally prescribed by a representation of the desired field, the so-termed gradient demand. This is a frequently numerically stored representation of the desired variation of the magnetic gradient field as a function of time. Due to a variety of causes, the actually generated gradient field differs in shape from the gradient demand. An important cause for this deviation is the temperature of the gradient coils in the gradient system, which may differ from the assumed temperature due to ambient influences and due the previous history of the use of these coils.
In said Japanese patent document, the temperature of a heat shield in the environment of the gradient system is measured at regular intervals during an MRI scan. Said temperature thus forms a quantity representing the effective conduction state of the gradient system, the term effective conduction state being taken to mean the electric conduction as it manifests itself in all current-conducting parts of the gradient system, such as the gradient coils, shielding coils (if any) and other metal parts which influence, due to eddy current induction, the formation of the magnetic gradient field. Said temperature is used as an input quantity to determine a correction in the form of a change of the eddy current compensation volume. In this known method, the temperature is measured of a part which is only indirectly involved in the formation of the gradient pulses, as a result of which the effect of the variation in temperature of the gradient coil is insufficiently compensated for.
It is an object of the invention to provide a method of the type mentioned in the opening paragraph, wherein a better compensation for the variation in temperature of the gradient coil is achieved. To achieve this object, the method in accordance with the invention is characterized in that during the generation of the gradient pulses, a gradient delay is determined on the basis of at least the value of said representing quantity, during the gradient pulses the instantaneous value of the gradient demand is corrected for the then prevailing value of the gradient delay, and the instantaneous value of the gradient pulses is determined by the value of the corrected gradient demand.